With the progress of high integration or large power consumption of semi conductor elements represented by IC (integrated circuit) and the like, realization of electrical insulating materials having good heat dissipation properties has been demanded. In order to meet this demand, various high heat conductive substrates have been proposed. Among them, aluminum nitride ceramic substrates have excellent properties in heat conduction, heat expansion and electrical insulation and, hence, have most keenly been being attempted to put into the practical use.
However, the aluminum nitride ceramic substrate has a defect that the price is expensive. As the reasons why the cost is expensive, there may be considered the following reasons: (1) the price of aluminum nitride powder used as the starting material is expensive; and (2) high-temperature sintering is required.
Hitherto, aluminum nitride powders have been produced by direct nitriding of aluminum or carbon reduction of alumina. For example, in the direct nitriding of aluminum, it is difficult to produce high-purity aluminum nitride powders having a small particle diameter. On the other hand, in the carbon reduction, the reaction must be carried out at high temperatures, leading to an increase in production cost. As an improvement of the carbon reduction of alumina, there is proposed a method in which an aluminum source is mixed in a solution state. However, in this proposed method, the mixing is carried out in a suspended state but not in the order of molecule and, therefore, the reaction must still be carried out at high temperatures, leading to an expensive production cost, too.